Marine seismic exploration operations commonly include towing a seismic streamer behind a vessel. The seismic streamer includes data communications channels, power conductors, one or more strength members, and a number of sound-sensitive hydrophones or other sensors.
As the streamer is towed through the water during seismic operations, its primary function is to receive seismic signals at the sensors from subsurface geological structures, convert these seismic or acoustic signals to voltage signals, and transmit these voltage signals to a central receiver on board the vessel. Commonly, a number of hydrophones are coupled together as a group, and the group of hydrophones is coupled to a conductor within the streamer at a takeout.
The hydrophones so coupled together receive the seismic signals and provide a weighted average of the signal received by the hydrophones in the group. This weighted average is provided in analog form to a conductor within the streamer. This signal is digitized and recorded for later display of the subsurface geological structures.
The sensors are designed for extreme sensitivity because the seismic signals are well attenuated by the time they reach the sensors. Unfortunately, the sensors are equally sensitive to all kinds of noise, both from the cable itself and external to the cable. For example, as the cable is towed through the water, the cable is subject to sudden sharp movements and other mechanical perturbations which are conducted through strength members through the cable. Any such noise may be conducted to a sensor, and is thus detected as a signal or can mask the actual seismic signal.
Linear fiber optic hydrophones are based on the phenomenon that external measurands alter some optical characteristic of the optical fiber sensor, such as its index of refraction or the optical path length for a light signal in the fiber. For most known structures, any axial strain in the cable translates to a concomitant strain in the linear optical fiber, particularly if the fiber is axially oriented, and thus axial cable stress alters the optical path length of the fiber, and spoils any detection scheme.
Various means have been used in the past to reduced the noise conducted to the sensors. Discrete hydrophone elements have been mounted in a foam or a fluid volume to eliminate shear wave stresses, for example. However, when linear optical hydrophones are used, the geometry of the cable for encapsulating the optical hydrophone becomes problematic.
Aside from the mechanical geometry of the cable for retaining a linear optical sensor, a technique for interrogating the sensors must also be included in the streamer system. A structure and a technique for efficient multiplexing in an interferometric sensor array of a large number of sensors was disclosed in parent application U.S. patent application Ser. No. 09/169,252, filed Oct. 9, 1998, and incorporated herein by reference. In a preferred embodiment, the structure defines an apparatus for interferometric sensing comprising an optical source, a tunable filter, a depolarizer for depolarizing optical radiation emitted by the optical source, a matched interferometer, a sensing interferometer, and a detector. The matched interferometer contains a phase modulator and the optical path length difference in the sensing interferometer is approximately equal to the optical path length difference in the matched interferometer. The optical source may be a broad band source or light, or a narrow band source such as a laser.
Thus, the structure disclosed in U.S. patent application Ser. No. 09/169,252 provided wavelength-addressable interferometers containing optical fiber Bragg grating pairs as reflectors. This configuration also dramatically reduced cross-talk between hydrophones, which is inherent in many architectures.
However, there remains a need for a structure whereby the fiber optic acoustic sensors can be incorporated into a seismic array. Such a cable structure should preferably include a linear optical fiber sensor wound around a compliant core or suspended in a spiral tube filled with a fluid. Alternatively, the sensor may include point optical hydrophones. Also alternatively, the linear optical fiber sensor may comprise a spiral structure around a fluid-filled open-cell foam to eliminate common sources of noise.